Optical tweezers have been used over the two past decades to probe biological objects of various sizes, from whole cells down to individual proteins. Force measurement devices based on double optical tweezers have initially been used to manipulate non spherical particles such as bacteria, and increasingly became an important tool for single molecule studies of nucleic acids, and their interactions with proteins.
An important feature of double optical tweezers derived from a single laser source is that, although the absolute position of each trap is sensitive to external mechanical perturbations, their relative position can be precisely imposed. Beam steering may be achieved with galvanometer, piezoelectric tilt mount or acousto-optic deflectors. The force acting on one bead is often measured with the back focal plane method, which allows decoupling the force signal from trap displacement, and hence external vibrations. The two traps usually exhibit perpendicular polarization in order to reduce interference as well as to easily discriminate between them for detection. A laser of different wavelength can be used for detection, but a parasitic signal may then arise from the relative drift between the trapping and detection lasers.
When one of the two trapping beams is used for force measurement, it has to be distinguishable from the second beam of the double trap. Orthogonal polarizations can be used for this purpose. However, when linearly polarized light goes through a system of microscope objectives, such as in an optical tweezers apparatus, it suffers form the rotation of polarization, resulting in a non homogeneous polarization when it exits the microscope. Consequently, important crosstalk may occur when force is measured in this configuration. This crosstalk limits the force resolution of the force measurements.